Location: Biological Control of Insects ResearchTitle: PGE2 induces oenocytoid cell lysis via a G protein-coupled receptor in the beet armyworm, Spodoptera exigua Author
Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2011
Publication Date: 11/1/2011
Citation: Shrestha, S., Stanley, D.W., Kim, Y. 2011. PGE2 induces oenocytoid cell lysis via a G protein-coupled receptor in the beet armyworm, Spodoptera exigua. Journal of Insect Physiology. 57:1568-1576. Interpretive Summary: Application of classical insecticides has introduced severe problems in agricultural sustainability. The concept of biological control of insects is a potentially powerful alternative to classical insecticides. Biological control is based on the idea that direct application of insect-specific pathogens and parasites can reduce pest insect populations and the economic damage due to pest insects. The problem, however, is the efficiency of these organisms in biological control programs is limited by insect immune defense reactions to challenge. One approach to improving the efficiency of biocontrol agents would be to somehow disable insect immune reactions to viral, bacterial, fungal and parasitic infections. With this goal, we are investigating how insect immune reactions to infection are signaled. In this paper we report on identification of biological signals that are responsible for stimulating insect defenses to infection. This new research will be directly useful to scientists who are working to improve the efficacy of biological control methods. The ensuing improved biological control methods will benefit a wide range of agricultural producers by supporting the long-term sustainability of agriculture.
Technical Abstract: Eicosanoids mediate cellular and humoral immune responses in the beet armyworm, Spodoptera exigua, including activation of prophenoloxidase (PPO). PPO activation begins with release of its inactive zymogen, PPO, from oenocytoids in response to prostaglandins (PGs). Based on the biomedical literature, we hypothesized that PGs exert their actions via specific G protein-coupled receptor(s) in S. exigua. This study reports on a G protein-coupled receptor (Se-hcPGGPCR1) gene, which is expressed in the hemocytes of S. exigua. The Se-hcPGGPCR1 consists of 415 amino acids and shares sequence homology with rhodopsin-type GPCRs. The high composition of hydrophobic amino acid residues within the Se-hcPGGPCR1 protein is explained by prediction of seven-transmembrane domains that are characteristic of these GPCRs. Except for the eggs, Se-hcPGGPCR1 was expressed in all life stages. During the larval stage, it was expressed in hemocytes and gut, but not in fat body nor in epidermis. Real time quantitative RT-PCR showed that bacterial challenge induced more than 20-fold increases in its expression level. Fluorescence in situ hybridization showed that Se-hcPGGPCR1 was expressed in a specific hemocyte type, the oenocytoids. A specific eicosanoid, PGE2, significantly induced oenocytoid lysis and increased PO activity in the plasma. In contrast, when Se-hcPGGPCR1 expression was suppressed by RNA interference (RNAi), the oenocytoid lysis and the PO activation in response to PGE2 were not elevated above basal levels. A binding assay using intracellular calcium mobilization showed that the RNAi-treated hemocytes were significantly less responsive to PGE2 than the control hemocytes. These results support our hypothesis with the specific finding that PGE2 acts through Se-hcPGGPCR1 to activate PPO by lysing oenocytoids.